Overall, I'd take the S5.
Yes looks wise the m8 wins, hands down. However, this is a pretty novelty-based criteria. One way or another you will get used to the phone's looks and disregard it. You won't, months after owning it, be saying OH MAN I CANT PUT IT DOWN ITS SO SHINY!!!. S5 has a superior camera, and battery which can be removed and changed if wanted.
Yes looks wise the m8 wins, hands down. However, this is a pretty novelty-based criteria. One way or another you will get used to the phone's looks and disregard it. You won't, months after owning it, be saying OH MAN I CANT PUT IT DOWN ITS SO SHINY!!!. S5 has a superior camera, and battery which can be removed and changed if wanted.